This invention relates to low pass RF filters and more particularly to a layer of dielectric material deposited on a ferrite substrate to form a filter.
Low pass RF filters are used extensively in electrical circuits to suppress stray radio frequency noise. Lumped impedance filters perform well at the lower frequencies but resonances limit their utility as the frequency is increased. Also, these type filters are large in size compared to the circuits with which they are used. To overcome this, RF filters of the type disclosed in U.S. Pat. No. 3,275,953 -- Coda et al. were developed and used as feed through filters or on connector pins. These filters are small and have good insertion loss characteristics at high frequencies. However, there are several problems associated with filters of the type shown in the Coda et al. patent. First, they include an inner sleeve of ferrite coated with a metal layer and an outer metallized ceramic sleeve, usually barium titanate. Therefore, they require several fabrication steps. Also, the capacity is limited by the thickness to which the outer sleeve can be made, usually 8 to 10 mils minimum.
Finally, even though resonances are minimized at high frequencies, the filter, because of the type of construction, is still .[.lumpted.]. .Iadd.lumped .Iaddend.at the lower frequencies of interest, 1-50 megahertz. Accordingly, resonances can result at these frequencies. It is desirable then that filters of this type have a completely distributed impedance, that they be easier to fabricate and that they not be limited in capacity by the titanate sleeve thickness.